1. Field of the Invention
The present invention relates to a transmission line and an antenna device including the transmission line.
2. Description of the Related Art
In an antenna device for a radio communication apparatus, such as a mobile terminal, a transmission line for transmitting high-frequency signals has been used, which forms a distributor or a multiplexer between a high-frequency power source and a transmitting/receiving device. Examples of the transmission line include a microstrip line and a triplate line, which is formed by a pair of outer conductors disposed in parallel with each other and a plate-like central conductor interposed therebetween (see, e.g., Japanese Unexamined Patent Application Publication No. 2003-264420).
A triplate line described in Japanese Unexamined Patent Application Publication No. 2003-264420 includes an inner conductor and a pair of outer conductors facing each other with the inner conductor interposed therebetween, the outer conductors being an outer conductor (first outer conductor) and a reflective plate (second outer conductor). The inner conductor is connected at one end to a feeding transformer in an antenna element, and is connected at the other end to a feeding unit. There is a space between the inner conductor and the first outer conductor, and between the inner conductor and the second outer conductor.
Characteristics of a triplate line of this type tend to be unstable, due to positional displacement of a central conductor or changes in distance between outer conductors. Therefore, particularly when the triplate line is relatively large in size, the central conductor needs to be supported between the outer conductors by a spacer made of an insulator, such as resin.
However, since such a spacer itself has a unique dielectric constant higher than that of air, an impedance in a supported portion of the central conductor supported by the spacer is lower than an impedance in the surrounding area. This results in an impedance mismatch and reflection of high-frequency signals.
As a measure to reduce reflections, the present inventors initially intended to narrow the line width of the supported portion of the central conductor. With this method, the supported portion itself exhibits an impedance higher than that of the surrounding area depending on the line width. Therefore, by setting the line width of the supported portion in consideration of the dielectric constant of the spacer, the impedance in the supported portion can be matched to the characteristic impedance of the triplate line in the surrounding area, and reflections can be reduced.
However, assume for example that the supported portion has a through hole passing therethrough in the direction of thickness of the central conductor, and that the spacer is secured to the supported portion by inserting part of the spacer into the through hole. In this case, the line width of the supported portion is extremely narrow in the area around the through hole. Because this may affect the mechanical strength of the supported portion, it has been difficult in practice to provide such a through hole in the supported portion.